There is a known technology that detects a vehicle traveling on a road based on data measured by a radar in order to collect information regarding, for example, traffic volume. A system to detect a travelling vehicle needs to avoid erroneously detecting a stationary body as a travelling vehicle. Here a stationary body may include a fixed object such as, for example, a guardrail or an object that does not move for a given period of time such as, for example, a stopped vehicle. Therefore, a technology is proposed that acquires background data indicating the position of a stationary object and detects a travelling vehicle by excluding the position of the stationary object, which is indicated by the background data, from the data measured by a radar. See, for example, Japanese Laid-Open Patent Publication Nos. 2000-172980 and 2002-99986.
For example, an obstacle detecting method disclosed in Japanese Laid-Open Patent Publication No. 2000-172980 performs processing to detect an obstacle candidate from a difference between scanning data obtained using a millimeter wave sensor installed by a road and background data that does not include an obstacle. The method then repeats the process and detects an obstacle when the obstacle candidate continues to be present. A traveling vehicle detecting method disclosed in Japanese Laid-Open Patent Publication No. 2002-99986 tracks a vehicle with every scan of a millimeter wave radar installed above the road and detects a travelling vehicle using the acquired tracking data. In the traveling vehicle detecting method, the data of a fixed object such as a guardrail is eliminated by subtracting the background data from the scanning data.
In particular, Japanese Laid-Open Patent Publication Nos. 2000-172980 and 2002-99986 disclose that the background data is updated by adding, to the background data, a value acquired by multiplying the gradation data prepared from the measurement data in each of the distances within a detection range of the radar by a forgetting coefficient.